1.1 Field of the Invention
The present invention relates to the use of either oil dispersible or oil soluble polynuclear aromatic compounds or functionalized lignin as additives to delayed coking feeds.
1.2 Description of Related Art
Delayed coking is a process for the thermal conversion of heavy oils such as petroleum residua (also referred to as “resid”) to produce liquid and vapor hydrocarbon products and coke. Delayed coking of resids from heavy and heavy sour (high sulfur) crude oils is carried out to convert part of the resids to more valuable hydrocarbon products. The resulting coke has value, depending on its grade, as a fuel (fuel grade coke), electrodes for aluminum manufacture (anode grade coke), etc.
In the delayed coking process, the feed is rapidly heated at about 500° C. (932° F.) in a fired heater or tubular furnace. The heated feed is conducted to a coking vessel (also called a “drum”) that is maintained at conditions under which coking occurs, generally at temperatures above about 380° C. (716° F.) and super-atmospheric pressures. Coke drums are generally large, upright, cylindrical, metal vessels, typically ninety to one-hundred feet in height, and twenty to thirty feet in diameter. Coke drums have a top portion fitted with a top head and a bottom portion fitted with a bottom head. Coke drums are usually present in pairs so that they can be operated alternately. Coke accumulates in a vessel until it is filled, at which time the heated feed is switched to the alternate empty coke drum. While one coke drum is being filled with heated residual oil, the other vessel is being cooled and purged of coke.
Typically in the cooling step, the filled drum is quenched with water to lower the temperature to a range of about 93° C. to about 148° C. (about 200° F. to about 300° F.), after which the water is drained. When the cooling step is complete, the drum is opened and the coke is removed by drilling and/or cutting. The coke removal step is frequently referred to as “decoking.”
In delayed coking, the coke is typically cut from the drum using a high speed, high impact water jet. A hole is typically bored in the coke from water jet nozzles located on a boring tool. Nozzles oriented horizontally on the head of a cutting tool cut the coke from the drum. The coke removal step adds considerably to the throughput time of the process. Drilling and removing coke from the drum takes approximately 1 to 6 hours, and the coker drum is not available for feed coking until the coke removal step is completed. This delay can unfavorably impact the yield of hydrocarbon vapor from the process. Thus, it would be desirable to be able to produce a free-flowing coke in a coker drum, which could be removed more speedily, thereby minimizing the expense and cycle time associated with conventional coke removal.
An additional difficulty that may arise in decoking results from non-uniform coke cooling prior to decoking, a problem sometimes called a “hot drum.” Hot drums occur when, following water quench, regions of the coke volume in the drum remain at a significantly higher temperature than other regions. Hot drum may result during cutting or drilling from the presence of different coke morphologies (e.g., shot and needle or shot and sponge) in different regions of the drum. As a result of the different thermal characteristics among the coke morphologies, some coke regions in the drum may differ in temperature significantly from other regions, which can lead to unpredictable and even hazardous conditions during decoking. Since free-flowing coke morphologies cool faster than agglomerated coke morphologies, it would also be desirable to produce predominantly free-flowing coke in a delayed coker in order to avoid or minimize hot drums.